What is a sum in Excel?

SUM() is a function within Excel which adds up the values of the cells that are passed to it. You can pass individual cells, a list of cells or a range of cells. e.g.

=SUM(A1) returns the same value as the cell A1
=SUM(A1,B2) return the sum of A1 + B2
=SUM(A1:B2) returns the sum of the range A1:B2

Ranges are effective squares of cells, so the range A1:B2 is a small 2x2 square consisting of A1, A2, B1 and B2.